Scanning Electron Microscopy and Energy Dispersive Spectroscopy Reveal Amorphous Apatitic Cores in Ductal plugs bearing Calcium Oxalate Stone Overgrowths

Kidney stone prevalence is increasing, with high recurrence rates and associations with comorbidities such as metabolic syndrome and chronic kidney disease. The most common stones in nephrolithiasis are calcium oxalate (CaOx) stones, either monohydrate (COM), dihydrate (COD) or both, which can form as overgrowths on intratubular plugs of mineral that protrude from the terminal collecting ducts of the renal papilla. Although this is a recognized mode of stone formation, the underlying mechanisms remain poorly understood. In this study, we analyzed 11 ductal plug stones from four patients using stereoscopic microscopy, micro computed tomography (micro CT), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Detailed surface imaging revealed COM and COD overgrowths on apatite plugs, which often showed various crystallized phases and typically exceeded normal ductal luminal diameters (mean = 575 ± 440 µm). Sectioned plugs revealed amorphous nonconcentric apatitic cores and radially expanding mineral layers suggestive of mineral accretion. These findings support a model in which ductal plugs originate within the collecting duct lumen and grow asymmetrically beyond the duct wall, potentially contributing to epithelial injury and crystal retention. Further investigations using advanced imaging, molecular, and omics approaches are needed to elucidate the pathophysiology of plug formation and stone retention at the renal papilla.